Layers can cover up. If the patient can also has the effect of oil painting, but dark pigments are wet, dry color becomes shallow. With such as rosy purple, violet these color easy to interpret. So you don't have to render. Gouache color very fine grain, with water soluble after color is very beautiful, but it can't cover the base. Watercolor of the group of colors such as green, red ochre, soil mineral pigment, used alone or in combination with other color mix are prone to precipitation phenomenon.